【作者】 徐伟；

【导师】 黄光胜；

【作者基本信息】 重庆大学 ， 材料加工工程， 2008， 硕士

【摘要】 镁合金是目前工程应用中最轻的金属结构材料,具有高比强度、良好的导热性和减震性等特点,被认为是21世纪最富于开发和应用的“绿色材料”。但是到目前为止,由于镁合金的塑性加工性能差的问题没有得到根本解决,使其应用范围不够广泛。目前采用镁合金加工的零部件多数是通过压铸的方法加工而成的,如照相机、手机、笔记本电脑等产品的外壳。为了使镁合金的应用范围更加广泛,用板材成形方法加工零件是最佳的选择,不仅可以充分利用镁合金材料优异的机械性能,而且能够大幅度提高生产效率和产品合格率。因此镁合金板材的冲压成形性能和工艺的研究是扩大镁合金应用的关键所在。冷轧退火态镁合金板材具有很强的(0002)基面织构,室温下滑移系少,形变各向异性强,不利于冲压成形。目前,镁合金冲压工艺方面已有一定的研究,但对如何改善、提高镁合金板材冲压性能的研究还较少。本文提出了一种改善镁合金板材织构与冲压性能的新方法——单向多道次弯曲。自制单向多道次弯曲实验设备,以镁合金板材杯突值为指标考察成形性能,通过正交实验确定变形中的工艺参数,获得适合改善镁合金板材成形性能的优化变形方案。以优化工艺为指导,对冷轧退火态AZ31B镁合金薄板进行单向多道次弯曲变形,分析变形前后各状态样品的显微组织、力学性能、冲压性能及织构状态。结果表明,单向多道次弯曲工艺使晶粒在变形过程中发生转动,大幅削弱对后续塑性成形不利的(0001)基面织构组分,出现了很强的(1212),(1211),(0110)等面织构。同时还有少量的(0112),(0111)组分。在135℃回复温度下退火1h,(0112),(0111)组分增强,织构组分变得更为复杂多样,且此时出现大量退火孪晶。在再结晶温度260℃下退火1h,板材表面附近晶粒因发生静态再结晶而长大,主要织构为(1212),(1211),(0112),(0111)等锥面织构组分,由于定向形核及核心生长,各组分对应的晶粒取向趋于集中。板材在拉压变形时基面Schmid因子增大,基面滑移系在常温下更容易启动。由于织构状态的改变,在轧制方向(单向多道次弯曲方向)上,板材强度下降,屈强比降低,延伸率最高增加38%。单向多道次弯曲后,板材的各向异性较变形前有一定程度的增大,杯突值最高能增加67%。单向多道次弯曲工艺很好地改善了冷轧退火态AZ31B镁合金板材的织构组分,使成形性能大幅度提高。此外,本文还对中高温下单向多道次弯曲工艺进行了初步探索。发现中高温时板材基面织构同样被削弱,但变形过程中发生的动态再结晶使晶粒变得异常粗大。粗大的晶粒成为制约板材成形性能提高的主要因素,使得该工艺对镁合金板材成形性能的改善比较有限,低于常温下的提升幅度。更多还原

【Abstract】 Magnesium alloys, the lightest constructional metallic material with high strength, good thermal conductivity and the characteristics of the shock absorber, are considered as 21 century’s green materials. However, the applications of magnesium alloys, specially wrought magnesium alloy, are still limited due to their poor formability. Most magnesium alloys were manufactured into electronic product shells by die casting, such as cameras, cell phones and laptops. If these shell products were made by stamping using magnesium alloy sheet, it can not only make full use of excellent mechanical properties of magnesium alloy, but also substantially improve production efficiency and qualification. Thus, research on formability and process of magnesium alloy sheet is the key to expand the application.Presently, research of magnesium alloy stamping process has got some achievement, but report on how to improve formability of magnesium alloy is rare. Cold rolled magnesium alloy sheet has strong (0002) textural component, less slip systems and strong anisotropy at room temperature which are harmful to formability. In this paper, a new method, repeated unidirectional bending (RUB) was used to improve the textural components of cold rolled magnesium alloy sheet in order to obtain better formability.Focused on development of formability of magnesium alloy sheet, the technological parameters were determined by orthogonal experiment using a homemade RUB equiqment so that optimal RUB method was acquired. Cold rolled AZ31B magnesium alloy sheet was bended using the optimal method mentioned above. By analyzing microstructures, mechanical properties, Erichsen value and textural components of the samples, it was found that grains had rotated after RUB which demonstrated textural components evolution. The strong (0001) textural component which was harmful to post forming had been greatly weakened. And there was no obvious change on microstructure. Lots of ( 1 2 1 2),( 1 2 1 1)and (01 1 0)and a few(01 1 2),(01 1 1)textural components appeared.When annealed at 135℃for an hour, massive annealing twins apperared and(01 1 2)(,01 1 1)components increased. The textural components presented complex and diverse. When annealed at 260℃for an hour, grains near to sheet surface grew up through static recrystallization. Here main textural components were( 1 2 1 2),( 1 2 1 1),(01 1 2),(01 1 1). Grain orientation of corresponding texture focused because of nucleation and directional core growth. Schmid value of basal plane increased when sheet was under tension or compression deformation. Therefore, basal slip system became easier to start. Owing to the change of textural components, strength and yield ratio of the sheet reduced and elongation increased to 38% at most. Anisotropy of the sheet became more obvious after RUB while Erichsen value dramaticly increased to 67% at most. As we can see, RUB can well improve formability of cold rolled AZ31B magnesium alloy sheet.Moreover, in this paper, intermediate and high temperature RUB process was also researched preliminarily. Result indicated that (0001) textural component was still weakened, but grains coarsened due to dynamic recrystallization during deformation. Grain size became the chief factor of formability. Thus, the development of magnesium alloy sheet formability of intermediate and high temperature RUB was limited. The RUB process at normal temperature is the better.更多还原